The present invention relates to cutting instruments employing wedge-shaped cutting edges such as axes, knives, chisels and especially razor blades, said cutting edges being formed of diamond which has been shaped by contacting it with a moving, diamond surface under conditions of pressure and velocity sufficient to thermally convert a portion of the diamond on the cutting edge to softer forms of carbon.
The formation of cutting edges on steel razor blades conventionally involves a series of grinding and honing operations to produce a sharp and relatively durable shaving edge. Each grinding operation forms a facet on the blade edge being sharpened, which facet is modified by subsequent grinding operations of increasing fineness. Generally, the finished blade edge is wedge-shaped, having an included solid angle about 18.degree.-26.degree.. The faces or sides of the cutting edges may extend back from the ultimate edge a distance of up to 0.1 inch or even more. Each face is typically made up of two or more facets formed by the successive grinding and honing operations recited above. The final facet, i.e. the facet immediately adjacent the ultimate edge has a width as low as 7.5 microns or even less compared to the diameter of beard hair which ranges from 100-125 microns The steel of which the blade edge is composed may be either carbon steel or stainless steel In either case it is hardened by a suitable process, as by heat treating or working.
During the honing of the final facet, deflection of the steel blade strip in the sharpening machine together with the mechanical interaction between the steel and the abrasive particles of the grinding wheel produce a final facet which is usually not planar but slightly convex. The curvature is a function of the type of steel and grinding wheel employed, as well as the setting parameters of the sharpening machine. Because of the resulting convexity of the final facets on each side of the blade, the blade tip cross section of the ultimate edge is customarily referred to as "Gothic arched". Through shave test evaluation and measurement of the geometry of such sharpened cutting edges, it has been found that the ultimate edge should have an average tip radius of less than about 500 angstroms. Typically, a shave facilitating layer of an organic polymeric material is applied to the area of the blade adjacent the ultimatef edge. Useful materials are described, inter alia, in U.S. Pat. Nos. 2,937,967; 3,071,858 and 3,071,856.
More recent studies have shown that the shaving performance of razor blades can be significantly improved if the thickness of the cutting edge over a distance back from the ultimate edge is substantially less than that of traditionally manufactured blades. The minimum useful thickness of the blade over the first 40 microns back from the ultimate edge is determined by the nature of the blade material. For example, for the types of steel currently used, the blade needs to be at least about 0.7 micron thick at a distance of about 1.0 micron from the blade edge in order to have sufficient strength to withstand the bending forces on the edge occurring during the shaving process, forces which can cause the edge to deform plastically or fracture in a brittle fashion, depending on the mechanical properties of the blade material.
In an attempt to develop blades of reduced thickness over the first 40 microns back from the ultimate edge, the prior art suggests the use of harder blade materials such as titanium carbide, boron nitride and diamond. U.S. Pat. No. 4,720,918 states that in the case of diamond, for example, blade thicknesses in the critical region would be approximately 40% of those calculated for stainless steel. The patent, however, does not describe a method for manufacturing a blade having the thinner cutting edges which should be obtainable with harder materials such as diamond or diamond-like carbon.
A major problem is, of course, posed by the need to sharpen a diamond coated blade to the dimensions and blade profile discussed above. Since the only material hard enough to abrade a diamond surface is another diamond, it is necessary to employ diamond surfaced grinding tools or lapping processes using progressively finer sizes of diamond grit. Since the smallest partical grit size for abrading diamond is about one micron, a typical well-polished diamond surface exhibits one-half micron scratches. Such a result is unsuitable for the formation of an ultimate edge having an average tip radius of less than 500 angstroms.
PCT International Publication No. WO 87/04471 describes a method of forming such cutting edges by coating a preexisting cutting edge formed of steel or other substrate material with a material such as diamond, the coating being accomplished by a vapor deposition process. Simultaneously, the cutting edge is subjected to ion bombardment with ions of sufficient mass and energy to cause sputter removal of the deposited material at a rate which is less than the rate of deposition, thereby forming a cutting edge of diamond having the desired geometry.
PCT International Publication No. WO 90/03455 describes a method of forming or modifying the cutting edge of razor blades such as steel blades as they are being coated with diamond-like carbon on both sides of the edges.
A number of other non-abrasive methods for working diamond surfaces have been described in the prior art including the use of laser energy in a variety of applications. Among these are U.S. Pat. Nos. 3,527,198; 4,392,476; and 4,401,876. In U.S. Pat. No. 2,931,351 there is described a method of polishing a diamond by subjecting its surface to the action of a first flame having an excess of acetylene to heat it followed by subjecting it to the action of a second flame having an excess of oxygen to glaze the surface. Finally, U.S. Pat. No. 4,662,348 describes a method for burnishing complementary, conical, polycrystalline diamond bearing surfaces by rotating them at sufficient pressure and velocity to polish the bearing surfaces. None of these processes is directed at producing a result having the sub-micron dimensions required for the forming of a suitable razor blade cutting edge as described above.